Help switching High powered LED circuit

[stl_jones@yahoo.com]

I finally got my circuit that gives a high (12V) output when a low frequency is on the input.

I've also found this circuit on instructables which does a good job of driving my high powered LED's. However, I'm having a little trouble switching this circuit. **broken link removed**

I've tried using a N channel power MOSFET to switch R3 to ground, but this is causing a slight problem because I am having trouble because the voltage on the drain is slightly higher than ground.

I think the best way to switch this circuit would be to just switch the LED's. I was thinking of hooking up a N channel MOSFET in between the cathode of the LED's and the drain of the MOSFET in the current control circuit. I'm not sure an N channel MOSFET is meant to be used this way, and I'm not sure how to use a P channel MOSFET for high-side switching applications. Thanks

 

[crutschow]

Switching R3 to ground will cause to LED current to increase significantly, since you are taking the constant current part of the circuit out of the loop.

All you need to do to turn off the LEDs is switch the gate of Q2 to ground, either using an N-MOSFET or an NPN bipolar transistor such as Q1. If you use an NPN be sure and place a resistor in series with the base (10k should work) for the turn-off signal.

 

[stl_jones@yahoo.com]

Ok, wow, can't believe I didn't see that myself, good idea.

One more question. I have a decent sized heatsink on Q2. I'm under the assumption that Q1 and this new N-MOSFET won't dissipate much power and won't need heatsinks, correct?

 

[stl_jones@yahoo.com]

Since I want the LED's to be ON when the output signal is high, I would need a JFET transistor that would turn off when the input signal goes high, correct? The only JFET they have at radioshack is this one.

https://www.radioshack.com/product/index.jsp?productId=2062594&CAWELAID=107598742

It's a JFET op amp??? What's that and will it work for this?

 

[KeepItSimpleStupid]

A JFET is a Junction FET, J174, J175 and J176 are typical type numbers. There is a theshold voltage when Vgs is exceeded for which it will start to turn off.

A JFET input OP amp uses these at it's input in a differential configuration because of their low current requirements. A JFET in a 3 terminal device.

 

[crutschow]

Since I want the LED's to be ON when the output signal is high, I would need a JFET transistor that would turn off when the input signal goes high, correct? The only JFET they have at radioshack is this one.

https://www.radioshack.com/product/index.jsp?productId=2062594&CAWELAID=107598742

It's a JFET op amp??? What's that and will it work for this?

An op amp could be used but it's overkill for this requirement.

What is the nature of your control signal (volts, current available, etc.)? If it's 12V, then you could just connect the top of R1 to the signal. That would turn Q1 on when the signal was 12V and turn it off when the signal is low (near ground).


Q1 and any added control circuit will dissipate only a small amount of power, no heatsink needed for either.

 

[stl_jones@yahoo.com]

Good idea, I'll give that a try also.

The signal is filtered from audio. It gives a high(6V-12V) when there is a low frequency and ~0V when a low frequency is not present.

 

[KeepItSimpleStupid]

I should also say that you could configure the OP amp as a comparitor, providing it's a single supply OP amp and it doesn't have to be that one. AT most three additional resistors and a diode might be required.

 

[crutschow]

Good idea, I'll give that a try also.

The signal is filtered from audio. It gives a high(6V-12V) when there is a low frequency and ~0V when a low frequency is not present.

For the circuit to operate properly the high voltage must be at least equal the maximum Vgs threshold of Q2 (from its data sheet) plus 1V. For that, 6V may not be enough.

 

[stl_jones@yahoo.com]

Not quite sure what your last post means. I am using a IRF150 N-MOSFET. The maximum Vgs of Q2 is 20V. However, the circuit works perfectly switching R1.

 

[crutschow]

I'm referring to the maximum Vgs threshold voltage (the voltage where it just starts to turn on), not the maximum allowable Vgs. From the data sheet the maximum threshold Vgs is 4V. Thus the gate voltage needs to be over 5V to supply the required LED current (>4V Vgs + 0.7V across R3). 6V applied to R1 should thus likely be enough to operate the circuit normally.

What do you mean "switching R1".

 

[stl_jones@yahoo.com]

I think my terminology is off.

I mean I am using the output signal to switch one side of the resistor from 0V to 6-12V.

As in my post about switching R3 to ground; what I meant was I was using a MOSFET to switch R3 to ground --- Not replacing R3 with a direct short to ground.

Thanks for all your help, it works great.

 

[crutschow]

OK. Glad it's working.